At present, when a voltage of a power grid works normally, an electrical apparatus (e.g., an air conditioner) having a grid interconnection function may accurately calculate an angle of the voltage of the power grid. However, in a condition in which the voltage of the power grid drops instantaneously, the calculation on the angle of the voltage of the power grid has a large deviation. A low voltage ride-through function is an important index to internationally determine the performance of a grid interconnection device. In the related art, an electrical apparatus designed with a traditional inverter as a rudiment does not possess the low voltage ride-through function. FIG. 1 is a schematic diagram of a photovoltaic air-conditioning system according to the related art. As shown in FIG. 1, a line voltage is collected for control. When three phases drop symmetrically, the three phases have already been asymmetric, the line voltage has large negative-sequence and zero-sequence components and then a certain limitation is formed to determine a drop amplitude of the voltage. A three-phase three-wire system is adopted by a working electrical apparatus, and a switching power supply is directly originated from a direct current and is controlled by using an inter-wire voltage. When three phases drop asymmetrically, a line voltage has large negative-sequence and zero-sequence components, and thus a large deviation is occurred to determine a drop amplitude of a voltage.
Concerning the above problems, an effective solution hasn't been proposed yet till now.